Analytics · Mass spectrometry

[2026 update] Why mass spectrometry cannot tell you about an epimer

Analytics › Mass spectrometry high-resolution MS counter-ion content MALDI-TOF

g.tanakaTL3Regular22 Nov 2025#1

Why mass spectrometry cannot tell you about an epimer — that is the question, and I have not found it answered plainly anywhere I have looked.

Working through the identity arithmetic and I would like it checked.

retatrutide has a monoisotopic mass close to 4731.3 Da. On an electrospray instrument I would expect to see the multiply charged series rather than the intact singly charged ion, so for the doubly charged species I calculate (4731.3 + 2 x 1.00728) / 2, and for the triply charged the analogous expression.

The observed values in the report sit within a few ppm of those. My question is what that actually establishes, because I have seen people treat a mass match as a purity result and I do not think it is one.

9 likes 8mo

a.pereiraTL2 Moderator2 Dec 2025#2

On the opening post — agreed on the reasoning, with one qualification.

Electrospray ionisation produces multiply charged ions. For a 4 kDa peptide you expect mostly 2+, 3+, and 4+ charge states. Reading an electrospray spectrum means recognizing the envelope, not looking for one peak.

13 likes 8mo

forest_plotTL3Evidence synthesis8 Dec 2025#3

a.pereira, post #2: On the opening post — agreed on the reasoning, with one qualification. Electrospray ionisation produces multiply charged ions. For a 4 kDa peptide you expect mostly 2+, 3+, and 4+ charge states. Reading an electrospray spectrum means recognizing the envelope, not looking for one peak. Go to post

Quantitation by MS: most quantitation is done by LC-UV detection at 214 nm, not by MS, because extinction coefficients are better known. MS can quantify if an internal standard is used but that requires preparation.

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About the Mass spectrometry categoryAnalytics › Mass spectrometry · 9 replies

Moved from Home & field testing by bench_notes. Category placement is not obvious from outside and getting it wrong is expected. This topic will get better answers here. The move is recorded in the public log citing R7.

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